Conventionally, when the color of image data acquired by an input device is faithfully reproduced by an output device, colorimetric color reproduction is executed to make input tristimulus values coincide with output tristimulus values using metamerism on the basis of the three color theory. Examples of the input device are a digital camera and scanner. Examples of the output device are a display, projector, and printer.
Generally as a space which quantifies tristimulus values, the CIE-XYZ colorimetric system and CIE-Lab colorimetric system which are defined by the CIE (International Commission on Illumination) are adopted. FIG. 13 shows the flow of a general colorimetric color reproduction system.
As shown in FIG. 13, an image acquired by an RGB input device such as a digital camera is processed in the XYZ color space or L*a*b* color space which is device-independent, and the processed data is output from an RGB (CMY) output device such as a display or printer. However, the XYZ values and L*a*b* values are influenced by the illumination light source. Even if the XYZ tristimulus values or L*a*b* values of an original color and the reproduced color coincide with each other under a given illumination light source, these values may become quite different from each other under another illumination light source. That is, the original color and reproduced color may be perceived to be different from each other.
In the field of color management and the like, implementation of spectral color reproduction is desired as a more faithful color reproduction method. In spectral color reproduction, the spectral reflection factor of an object that does not contain any illuminance light source is reproduced as color information. For this reason, the reproduced color coincides with the original color even under an arbitrary illuminance light source.
As a technique of implementing the spectral color reproduction, a multi-band input/output technique is expected to be promising. Colorimetric color reproduction is color reproduction based on three primary colors such as R, G, and B, or C, M, and Y, whereas the multi-band input/output technique is a technique of acquiring color information of an object using four or more primary colors and reproducing the spectral reflection factor. As an input device, a multi-band camera having four or more bands has been developed (Japanese Patent Laid-Open No. 2001-086383). As an output device, a multi-band output device has also been developed by developing a multi-primary color projector and the like and putting a multi-primary color printer into practical use (Japanese Patent Laid-Open No. 2001-054131).
FIG. 14 shows the flow of a spectral color reproduction system using a multi-band color input/output device.
As shown in FIG. 14, in the spectral color reproduction system, a spectral image having a spectral reflection factor at each pixel is acquired from an object by a multi-band input device. In the spectral space independent of a device and illuminance light source, the image is saved, processed, and transmitted. Finally, the spectral image is reproduced by a multi-band output device.
In the colorimetric color reproduction system, when a color outside the color gamut of an output device is input via an input device, it must be reproduced with a color falling within the color gamut of the output device (this processing will be called mapping hereinafter). Many proposals of the mapping technique have been made in the colorimetric color reproduction system.
For example, Japanese Patent Laid-Open No. 2000-287096 proposes mapping into the color space of a monitor serving as an input color space and into the output color space of a color printer. In Japanese Patent Laid-Open No. 2000-287096, colors displayed on the monitor are converted into a three-dimensional color space of lightness (L), color saturation (C), and hue (H). The lightness of the monitor is compressed into the lightness range of the printer, and colors are mapped at minimum points in the LC space for each hue.
In the spectral color reproduction system, even if the spectral reflection factor is sampled at an interval of 10 nm in a visible light region of 380 nm to 730 nm, this results in data of 36 dimensions for one color, and the data amount to be processed becomes very large. In order to completely reproduce and output the spectral reflection factor of an input, all data of 36 dimensions must be equal to each other. In practice, except for special circumstances, it is very difficult to equally reproduce all data of 36 dimensions. That is, processing for data of 36 dimensions targets colors outside the color gamut of the output device. It is, therefore, a task of the spectral color reproduction system to reduce the data amount and map a color to one having a similar spectral reflection factor.
In order to achieve this task, there is known a method of reducing the data amount of the spectral reflection factor and processing data in a multi-dimensional space of principal component coefficients. According to this method, spectral reflection factor data generally undergoes principal component analysis, and the spectral reflection factor data is expressed by principal component vectors and their addition ratio (to be referred to as a principal component coefficient). In this case, mapping processing in the colorimetric color reproduction system can be applied to the multi-dimensional space of principal component coefficients. Japanese Patent Laid-Open No. 2002-254708 discloses a technique of changing the principal component coefficient of each dimensional number at equal intervals in the principal component coefficient space. According to this technique, a so-called Look-Up-Table (to be referred to as an LUT hereinafter) is created from corresponding output signal values as a conversion table for converting a principal component coefficient into an output signal value. A principal component coefficient is input and converted into a corresponding output signal value to output the signal value.
A case wherein mapping processing in the three-dimensional space used in colorimetric color reproduction as described in Japanese Patent Laid-Open No. 2000-287096 is expanded to n dimensions (n>3) of the principal component coefficient space will be considered. In this case, the three-dimensional color space used in colorimetric color reproduction is a uniform color space. To the contrary, in the principal component coefficient space, contributions exist in respective dimensions, as shown in FIG. 4, and the information amounts in these dimensions are different from each other. For this reason, no high-precision mapping is achieved by simply applying the mapping method in colorimetric color reproduction.
In Japanese Patent Laid-Open No. 2002-254708, a long time is taken to create an LUT because all output signal values must be searched for output signal values corresponding to principal component coefficients arranged at equal intervals, in order to create an LUT. This reference does not specify how to process a color when a color outside the output color gamut is input.